On Mon, 25 Apr 2022, Yosry Ahmed wrote:
From: Shakeel Butt <shakeelb@xxxxxxxxxx> Introduce a memcg interface to trigger memory reclaim on a memory cgroup. Use case: Proactive Reclaim --------------------------- A userspace proactive reclaimer can continuously probe the memcg to reclaim a small amount of memory. This gives more accurate and up-to-date workingset estimation as the LRUs are continuously sorted and can potentially provide more deterministic memory overcommit behavior. The memory overcommit controller can provide more proactive response to the changing behavior of the running applications instead of being reactive. A userspace reclaimer's purpose in this case is not a complete replacement for kswapd or direct reclaim, it is to proactively identify memory savings opportunities and reclaim some amount of cold pages set by the policy to free up the memory for more demanding jobs or scheduling new jobs. A user space proactive reclaimer is used in Google data centers. Additionally, Meta's TMO paper recently referenced a very similar interface used for user space proactive reclaim: https://dl.acm.org/doi/pdf/10.1145/3503222.3507731 Benefits of a user space reclaimer: ----------------------------------- 1) More flexible on who should be charged for the cpu of the memory reclaim. For proactive reclaim, it makes more sense to be centralized. 2) More flexible on dedicating the resources (like cpu). The memory overcommit controller can balance the cost between the cpu usage and the memory reclaimed. 3) Provides a way to the applications to keep their LRUs sorted, so, under memory pressure better reclaim candidates are selected. This also gives more accurate and uptodate notion of working set for an application. Why memory.high is not enough? ------------------------------ - memory.high can be used to trigger reclaim in a memcg and can potentially be used for proactive reclaim. However there is a big downside in using memory.high. It can potentially introduce high reclaim stalls in the target application as the allocations from the processes or the threads of the application can hit the temporary memory.high limit. - Userspace proactive reclaimers usually use feedback loops to decide how much memory to proactively reclaim from a workload. The metrics used for this are usually either refaults or PSI, and these metrics will become messy if the application gets throttled by hitting the high limit. - memory.high is a stateful interface, if the userspace proactive reclaimer crashes for any reason while triggering reclaim it can leave the application in a bad state. - If a workload is rapidly expanding, setting memory.high to proactively reclaim memory can result in actually reclaiming more memory than intended. The benefits of such interface and shortcomings of existing interface were further discussed in this RFC thread: https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@xxxxxxxxxx/ Interface: ---------- Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to trigger reclaim in the target memory cgroup. The interface is introduced as a nested-keyed file to allow for future optional arguments to be easily added to configure the behavior of reclaim. Possible Extensions: -------------------- - This interface can be extended with an additional parameter or flags to allow specifying one or more types of memory to reclaim from (e.g. file, anon, ..). - The interface can also be extended with a node mask to reclaim from specific nodes. This has use cases for reclaim-based demotion in memory tiering systens. - A similar per-node interface can also be added to support proactive reclaim and reclaim-based demotion in systems without memcg. - Add a timeout parameter to make it easier for user space to call the interface without worrying about being blocked for an undefined amount of time. For now, let's keep things simple by adding the basic functionality. [yosryahmed@xxxxxxxxxx: worked on versions v2 onwards, refreshed to current master, updated commit message based on recent discussions and use cases] Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx> Co-developed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Acked-by: Wei Xu <weixugc@xxxxxxxxxx> Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
